(a) Technical Field
The present invention relates to an apparatus for controlling hydrogen supply of a fuel cell system and a method for controlling the same. More particularly, it relates to an apparatus for controlling hydrogen supply of a fuel cell system and a method for controlling the same, which can achieve a function of controlling hydrogen supply flow and anode recirculation performance necessary for a fuel cell in conjunction with a jet pump, by utilizing the flow control characteristics of a proportional control solenoid valve for control of hydrogen fuel supply to a fuel cell stack, and controlling the operation of the proportional control solenoid valve using a pulse flow control method together with a proportional control method.
(b) Background Art
A fuel cell system, a type of power generating system that can convert chemical energy into electrical energy, includes a fuel cell stack which is configured to generate electrical energy, a fuel supply system for supplying fuel (hydrogen) to the fuel cell stack, an air supply system for supplying an oxidant (oxygen) necessary for an electrochemical reaction to the fuel cell stack, and a cooling system for discharging reaction heat out of the system and controlling the operation temperature of the fuel cell stack.
Referring to FIG. 7, a fuel supply system connected to a fuel cell stack includes a hydrogen supply line 12 connected to a hydrogen storage tank 10, a hydrogen recirculation line 14 for recirculating unreacted hydrogen in the fuel cell stack, and a jet pump (i.e., an ejector) 16 installed at a point where a stack inlet 13 and the hydrogen recirculation line 14 intersect and configured to pump new hydrogen and recirculated hydrogen to an anode of the fuel cell stack. A stack inlet pressure sensor 18 installed at the stack inlet is configured to measure the pressure of hydrogen and air, and an ECU 22 is configured to control the flow control operation of a regulator 20 installed in the hydrogen supply line based on a signal detected and received from the stack inlet pressure sensor 18. The jet pump 16 may generate a vacuum by injecting compressed hydrogen supplied from a high pressure tank through a nozzle and may also recirculate hydrogen gas by suctioning exhaust gas in the fuel cell stack. Additionally, the system also includes a purge valve 25 connected to the recirculating line 14 for releasing excess hydrogen from the system.
Alternatively, as shown in FIG. 8, a blower 24 may be disposed in the hydrogen recirculation line 14 instead of the jet pump 16 as a mechanism or device for recirculating hydrogen.
Thus, in a conventional fuel cell vehicle, a blower or a jet pump may be used for smooth hydrogen fuel supply and recirculation. The objectives of hydrogen recirculation achieved by such a configuration lie in improving the system efficiency through fuel supercharging into an anode channel of a stack, improving the humidification efficiency by reloading humidified gas of a stack outlet into an stack inlet, improving flow uniformity in the stack according to an increase of the flow of the stack anode, and smooth supply of hydrogen gas fuel to a Membrane Electrode Assembly (MEA) through condensation water discharge of the stack anode.
On the other hand, in a system which adopts a blower like the one shown in FIG. 8, to recirculation hydrogen, must be equipped with a motor which is expensive, and bearings and other parts which may be easily corroded by condensation water of hydrogen recirculation gas thus, causing the blower to become corrupted. Particularly, when water form from condensation is frozen, it may cause a rotor in the blower to seize, thereby requiring the rotor to be melted by a heater.
Furthermore, a jet pump, as shown in FIG. 7, cannot generate a required recirculation flow due to a limitation of recirculation hydrogen fuel which is an energy source that can be used when a system load is small. That is, the jet pump is not able to generate the required recirculation flow when the load on the overall system is low due to the lack of hydrogen supplied to the system by the small load. That is, as shown in FIG. 9A, as hydrogen fuel flow (indicated as an arrow) supplied to a jet pump nozzle increases, the flow and pressure generated by the jet pump increase. Also, a crossing point between the flow of recirculated hydrogen and the system pressure drop curve corresponds to an operation point of actual hydrogen recirculation generated.
Regarding the hydrogen suctioning performance of a jet pump according to an increase of hydrogen fuel at a certain pressure, as shown in FIG. 9B, when the flow supply through a nozzle of the jet pump is small, the hydrogen injection pressure is low at the nozzle, and the flow velocity is significantly decreased. Accordingly, the suctioning pressure is not significant at this point, and thus the hydrogen recirculation flow suctioned by the jet pump is also reduced.
Accordingly, when there is a low load applied to the fuel cell system, thus requiring only a small amount of fuel to be used, the recirculation flow is not sufficient under the above operating conditions, and thus the operation state of the stack channel becomes worsened. Also, the stack efficiency and durability may become worsened as a result as well.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.